http://canlii.ca/en/bc/bclrb/doc/2012/2012canlii44734/2012canlii44734.html is the link to a recent LRB decision that provides good guidance on what the law is when a union and an employer settle a grievance and the grievor signs a release. According to the LRB, absent circumstances that prove undue influence or duress, the release signed as part of the settlement will bar a complaint of a breach of duty of fair representation against the union.